The invention relates to an aqueous nickel hydroxide paste of high flowability for the vibration filling of foam-structure and fiber-structure electrode plaques.
Industrial nickel oxide electrodes (the correct terms are nickel hydroxide electrodes or nickel oxide hydroxide electrodes) can be classified, according to their current collecting structure, as tubular electrodes, pocket plate electrodes, sintered nickel electrodes and fiber plaque electrodes. In button cells, a round electrode of pressed active material without a current collecting structure is used. Plastic bound electrodes have not achieved any relatively great importance. The so called "controlled microgeometry" electrode, in which layers of nickel hydroxide material are held between a multiplicity of perforated nickel foils, have also failed to achieve any dissemination.
In the case of tubular electrodes, the prefabricated nickel hydroxide is ground in powder form, and in the case of the pocket plate electrode, prefabricated compacts, so called briquets, are used. In the case of the narrow-pore sintered nickel plaque, the active nickel hydroxide material is formed in situ in the pores by precipitation from nickel salt solutions by chemical means (using alkalis) or by electrochemical means (cathodic polarization). The chemical precipitation process achieves the necessary concentration of nickel hYdroxide in the pores only by repeating the soaking and precipitation several times with intermediate washing and drying. The electrochemical process achieves the filling in one step, the residence time of the electrode plate in the salt bath being about 1 hour. However, the salt bath alters its composition and has to be discarded from time to time. Both precipitation processes are expensive but are indispensable for powder sintered plaque.
Foam-structure plaque and fiber-structure plaque have been used for about 15 years for supporting the active material. The are composed of metal only or they contain additionally the structure providing plastic or carbon basic body. The impregnation with active material is normally performed using a precipitation process, but descriptions have also been given of mechanical filling processes using prefabricated material which have become possible as a result of the substantially larger pore size compared with powder sintered plaque.
The first mention of vibration filling of foam and fiber plaque with dry pulverulent iron sulphide for lithium/sulphur cells is found in U.S. Pat. No. 3,933,520. Pulverulent dry nickel hydroxide is not suitable for a fluidized bed process of this type. At values of between 0.4 and 1 g/cm.sup.3, the bulk density is too low, the flow properties are unfavorable and the health hazard is high. Attempts have therefore already been made to avoid these disadvantages by using nickel hydroxide suspensions.
The suitability of nickel oxide electrodes is assessed, inter alia, by how much material has been accommodated in the electrode volume. In general, a range of 1.2 to 2.2 g of nickel hydroxide/cm.sup.3 of empty volume is regarded as suitable in the literature (for example, G. Crespy, R. Schmitt, M. A. Gutjahr and H. Saufferer in Power Sources 7, page 219 and page 225, Academic Press 1979). However, very high degrees of filling around 2 g/cm.sup.3 are not suitable because of the considerable expansion of the electrode. The density of nickel hydroxide (3.94 g/cm.sup.3) means that a paste (suspension, slurry) which is intended to achieve the specified range of 1.2 to 2.2 g of nickel hydroxide/cm.sup.3 in a single filling operation must have a proportion by volume of nickel hydroxide of at least 30.5%.
In German Published, Unexamined Patent Application (DE-OS) 2,427,421, the proposal was made to allow freshly precipitated nickel hydroxide suspended in a mother liquor to act on a horizontally mounted fiber plaque. The application of vacuum to the lower side of the plaque and the excitation by an ultrasonically driven vibrator electrode in the suspension promote the penetration of the nickel hydroxide into the carrier plaque. The filling is, however, inadequate since an additional soaking with nickel nitrate melt and a chemical preciPitation of further nickel hydroxide by alkali is carried out. This is not surprising since the nickel hydroxide is precipitated in very bulky form with considerable quantities of water of crystallization and anion residues and the necessary density and proportion by volume is far from being achieved in the suspension.
The process described in the U.S. Pat. No. 4,217,939 starts from commercially obtainable nickel hydroxide powder with 10% nickel powder added as a conduction aid. An aqueous paste is formed which has a dry material concentration of 17% by weight for which a proportion by volume of 34.4% by volume of nickel hydroxide can be calculated. A reticulate metal foam plaque is passed horizontally on a perforated plate over the paste container in which the paste is agitated by stirring and is forced upwards into and around the plaque while paste is spread into the plaque from above by doctor blading. Before impregnation, it is necessary to fill the pores of the plaque with water since the paste cannot otherwise be introduced into all the pores. If necessary, additional dry nickel hydroxide is applied to the upper side of the plaque in order to improve the filling result. From this information it is apparent that the transfer of the paste is not achieved with a homogenous working material. The changes in concentration (displacement of the pure water situated in the pores by paste, or additionally applying dry Powder) make it difficult to carry out the process continuously.
In one publication (W. A. Ferrando and W. W. Lee, Proc. of the 31st Power Sources Symposium 1984, page 177), the starting point is also prefabricated nickel hydroxide which is made up with ethylene glycol to form a paste. The specified ratio by mass of 1:3 results, after a conversion to percentages by volume, in a proportion by volume of nickel hydroxide of 8.6%. The paste (heavy cream) is rubbed into a nickel fiber plaque. After drying, the procedure is repeated in order to increase the loading. The necessity for this results from the nickel hydroxide content of the paste. In addition, the use of an organic fluid instead of water is uneconomical, it is necessary to recover the fluid during drying and, in addition, disposal problems may arise for the solvent vapors produced.
In the Japanese published specification No. (Kokai) 81-82,577, a paste composed of nickel hydroxide, cobalt hydroxide, methylcellulose, nickel-plated polyethylene fibers, nickel powder and water is applied to a nickel-plated perforated iron plate and calandered to the desired thickness after drying. The properties of the paste, in particular, the fiber content, make it unsuitable for filling the pores of foam-structure or fiber-structure electrode plaque.
According, the object of the invention is to provide an aqueous nickel hydroxide paste of high flowability for the vibration filling of foam-structure and fiber structure electrode plaque which makes it possible to fill said plaque completely in one operation.
These and other objects are achieved by a paste having a content of nickel hydroxide of about 30% to 50% by volume, a maximum particle size of about 0.04 mm, a plastic viscosity of about 0.1 to 1 Pa.s and a yield value of between about 10 and 120 Pa, and also a pH of between about 9 and 12, and a content of about 0.5% to 5% by weight, based on the nickel hydroxide content, of a dispersant selected from the group consisting of water-soluble salts of polyphosphoric acid, wherein the polyphosphoric acid has about 3 to 20 phosphorus atoms per molecule.
The high content of nickel hydroxide of about 30% to 50% by volume is necessary in order to be able to produce, in one filling step, electrodes which have the concentrations of active material and consequently capacity values per unit volume which have been achievable only with a plurality of steps by the methods hitherto introduced. The particularly preferred range is between about 35% and 45% by volume. Such a high concentration can only be achieved with the aid of specific, very effective dispersants.
Suitable dispersants are the water-soluble salts, in particular the alkali salts, of polyphosphoric acids or of di- and polyphosphonic acids and their derivatives. Particularly suitable are polyphosphates containing about 2 to 20 phosphorus atoms per molecule, and in particular, the polyphosphates containing about 16 to 20 phosphorus atoms in the molecule are preferred. Dispersants from the group comprising di- and polyphosphonic acids and their derivatives should not have more than about two carbon atoms per phosphorus atom per molecule since the conversion of the carbon atoms in the course of the electrode reactions would otherwise produce an intolerably high impurity content. Of this group, 1-hydroxyethane-1, 1-diphosphonic acid (HEDP) or aminotrismethylenephosphonic acid in the form of their alkali salts are particularly suitable.
The sodium and potassium salts of 1-hydroxyethane-1, 1-diphosphonic acid (HEDP) exhibit a stronger liquefying action in nickel hydroxide pastes than the polyphosphates, i.e. for the same viscosity, a paste containing an HEDP salt as dispersant contains more nickel hydroxide, or for the same nickel hydroxide content, a paste containing HEDP salt as dispersant has a lower viscosity. Hereinafter, the tetrabasic acid anion is abbreviated to HEDP and the hydrogen atoms written separately. While the free acid H.sub.4 (HEDP) produces very viscous paste, an optimum in the effect is between NaH.sub.3 (HEDP) and Na.sub.2 H.sub.2 (HEDP) The pH of the paste is about 10 or 11.2 and is consequently within the claimed range or between about 9 and 12. The pH of the paste containing Na.sub.3 H (HEDP) is already about 12.1. At this value, the absorption of CO.sub.2 from the air reaches a level which jeopardizes the use of the filled electrode in storage cells.
A surprising further increase in the nickel hydroxide concentration or reduction in the viscosity (reduction in the yield value or the plastic viscosity) is achieved by using an alkali-metal/cobalt complex of 1-hydroxyethane-1, 1-diphosphonic acid. Within the general formula Co.sub.x K.sub.y H.sub.z (HEDP), with 2x +y+z=4, the following ranges are permissible: x=about 0.5 to 1.25, y=about 0.5 to 1.5 and z=about 0.2 to 2. Outside these ranges, the pH is already too high and the yield value increases. The fluid can be produced by dissolving cobalt hydroxide in a suitable quantity in aqueous 1-hydroxyethane-1, 1-diphosphonic acid and adding a suitable quantity of alkali hydroxide.
The dispersant is used in quantities of about 0.5% to 5% by weight, based on nickel hydroxide, in particular in quantities of about 1% to 5% by weight. A higher addition only increases the number of foreign ions later present in the electrolyte without, however, achieving an improved effect. Below about 0.5% by weight, the effect of the dispersant is in some cases already too low and the viscosity of the paste is therefore too high.
In order to be able to penetrate the pores of the foam-structure or fiber-structure plaque, the paste must have an adequate flowability.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.